Summary of Work: An essential goal of our work is the isolation of DNA repair deficient fly strains. Signficant progress is being made towards obtaining flies lacking Rrp1 protein, a Drosophila repair enzyme with a putative major role in repair of alkylation and oxidative DNA damage. In collaboration with Jan Eeken, we have identified two strains with altered DNA repair capacity that are candidates for mutants in the Rrp1 gene. These strains are currently under study. In earlier studies, we demonstrated that overexpression of wild-type Rrp1 alters the amount of mutagen-induced somatic mutation and recombination resulting from oxidative DNA damage. Additional experiments demonstrate that overexpression of Rrp1 also alters spontaneous somatic mutation and recombination rates in animals lacking the oxygen radical-scavenger enzymes cSOD or catalase. In addition, the effect of Rrp1-overexpression on both spontaneous and mutagen-induced mutation and recombination rates is not seen in animals that overexpress a nuclease-deficient Rrp1 protein. These studies support the assertion that Rrp1-nuclease acts in flies to facilitate DNA repair, and suggest that nuclease-function is a rate-limiting step in base excision repair in Drosophila during the larval stage. Regulation and transcription of Rrp1 is being studied by analyzing the properties of promoter fusion genes constructed in vitro and transformed into flies. A 450 bp fragment containing promoter function was identified. The importance of putative promoter elements within this fragment are being examined by site-directed mutagenesis, and the role of additional upstream DNA sequence with additional promoter fusion genes.